Many of the current generation of unmanned aerial vehicles (UAVs) are electrically powered. Most electrically powered small UAVs are battery-powered, such as the RAVEN, WASP III, and PUMA AE by AeroVironment, Inc. of Monrovia, Calif., SCANEAGLE by Boeing of Seattle, Wash., and the MAVERIC UAS by PRIORIA ROBOTICS of Gainesville, Fla. The endurance (i.e., total flight time of the vehicle with a full battery charge) of the current generation of small, electrically-powered unmanned aerial vehicles (UAVs) is limited by power consumed by the UAV and the energy storage capacity of the battery. For example, the endurance of the RAVEN UAV is limited to approximately 90 minutes of flight time. The limit on endurance of small UAVs reduces the operational effectiveness of the small UAVs because it limits the time the UAV can spend over a target of interest, and limits a distance range for targets.
A High-Altitude Long Endurance (HALE) UAV is an airborne vehicle which functions optimally at high altitude (e.g., at least 30,000 feet or 9,000 meters above sea level) and is capable of flights which last for considerable periods of time (e.g., greater than 24 hours) without recourse to landing. Generally, recent generations of HALE UAVs are capable of operating at high altitudes and longer flight times than prior generations. Some examples of HALE UAVs are GLOBAL HAWK by Northrop Grumman Corp. of Falls Church, Va., ALTUS II by General Atomics Aeronautical Systems Inc. of San Diego, Calif., PHANTOM EYE by Boeing of Seattle, Wash., and ZEPHYR by Airbus Defense and Space of Farnborough, UK. Recently some HALE UAVs, such as ZEPHYR, have been produced that can fly at a maximum altitude 70,000 feet. For some types of HALE UAVs the need to refuel can set a limit on the maximum flight time or endurance of the UAV. For some types of HALE UAVs that are powered exclusively by solar cells, a reduction in the mass of the solar cells could increase the payload capacity of the HALE UAV.